Recently, an IPS (in-plane switching) mode (or horizontal electric field drive type) liquid-crystal display device, whose displaying is conducted by rotating the molecular axis direction (hereinafter referred to as `director`) of oriented liquid-crystal (hereinafter also referred to as `LC`) molecule in parallel direction to the substrate, has been researched and developed.
Such an IPS-mode LC display device does not have view-angle dependency to `standing direction` of LC molecule because only the short-axis direction is constantly viewed even when shifting its viewpoint. Therefore, it can obtain a wide view angle, compared with an LC display device, such as conventional TN (twist nematic) mode, where electric field is generated in the perpendicular direction of substrates to sandwich an LC layer between the substrates (hereinafter referred to as `vertical electric field drive type).
In the research and development of the IPS-mode LC display device, various techniques developed in the vertical electric field drive type devices are diverted and applied. However, the techniques of vertical electric field drive type devices cannot be unalteredly diverted thereto, particularly, as to view-angle characteristics and reliability, by the following reasons.
For example, with reference to normally-black LC display device, the comparison between TN mode as an example of the vertical electric field drive type and IPS mode intended for this invention will be explained.
In TN mode, directors are generally twisted by 90.degree. between two substrates in zero field, but they are existing in a plane parallel to the plane of substrate, like the case of IPS mode. However, in applying electric field, all directors are normally oriented in a plane parallel to the surface of substrate in IPS mode whereas all directors are normally oriented perpendicularly to the surface of substrate in TN mode.
Therefore, in IPS mode, the display appears to be white even when viewing from any viewpoint, but, in TN mode, it appears to be white or gray, as neutral color, depending on the viewing directions due to the refractive-index difference between the short axis and long axis of LC molecule. As understood from this, IPS mode and TN mode have no view-angle characteristic obtained resultantly in common, due to the difference in driving system.
Also, with respect to unit pixel composition, in TN mode, electrodes to compose a pixel are formed on two substrates, respectively because the electric field is generated perpendicularly to the plane of substrate, but, in IPS mode, all electrodes to compose a pixel are formed only on one substrate because the electric field is generated parallel to the plane of substrate. Namely, in TN mode, electric flux line to drive LC does not penetrate through the color layer of color filter, but, in IPS mode, electric flux line to drive LC penetrates through the color layer of color filter. Viewing from this, it is obvious that the degree of the influence of color layer of color filter to LC panel characteristics is different between TN mode and IPS mode.
From the differences described above, it is evident that the conventional techniques of TN cannot be unalteredly diverted to IPS.
FIGS. 1 and 2 are a cross sectional view and a top view, respectively, showing illustratively the basic composition of a conventional active matrix LC display device using horizontal electric field driving. Referring to FIG. 1, in this conventional LC display device, electrodes to compose a pixel electrode are formed only on one substrate 112 of two substrates sandwiching an LC layer 101 confined, on an opposing substrate 102 no electrode is formed and only a color filter to color light transmitted therethrough is formed.
Namely, on the electrode-forming substrate 112, one active element (not shown in FIG. 1), one drain signal electrode 103, one gate signal electrode (not shown in FIG. 1), and pairs of pixel electrodes (pixel electrodes 104 and common electrodes 105) are disposed in unit pixel. All formed on the color filter forming substrate 102 are color filter layers (Red 109, Green 110, Blue 111) to color light transmitted through LC into a specific color, generally red, green or blue, and a black matrix layer 108 to shield leakage light from the neighborhood of the drain signal electrode 103 on the electrode-forming substrate 112 or the gate signal electrode.
The color layers (R, G, B) 109, 110, 111 are formed considering the color purity and chromaticity level of light transmitted through panel. The color layers (R, G, B) 109, 110, 111 are produced by coloring an organic polymer material, such as polyvinyl alcohol or acryl resin, by using a dyestuff or pigment. Therefore, when producing it by, e.g., pigment scattering, the dielectric constants of the color layers (R, G, B) 109, 110, 111 vary, depending on the kind of pigment or the scattering density. Though the thickness H of color layer is set to be greater than 1 .mu.m so as to enhance the color purity, the respective thicknesses of the color layers (R, G, B) 109, 110, 111 are also different each other because the respective transmissivities of the color layers (R, G, B) 109, 110, 111 are different each other. Accordingly, the color-layer static capacitance of color layer represented by the product of color-layer dielectric constant and color-layer thickness H is not constant in the color layers (R, G, B) 109, 110, 111 each.
In the arrangement of respective electrodes within unit pixel, the common electrode 105 is located just nearby the drain signal electrode 103, the pixel electrodes 104 are disposed at certain intervals, the common electrode 105 and pixel electrode 104 are alternately disposed at equal intervals or unequal intervals, the common electrode 105 is further located in a layer covered with interlayer insulating film 106 nearby the substrate, different from the scanning signal electrode 103 and pixel electrode 104 (the common electrode 105 and pixel electrode 104 each are located in the separate layer).
Also, the active matrix LC display device uses AC drive to prevent the deterioration of panel members such as LC. For example, the polarity of signal is inverted every one field with a reference level (opposing electrode's level) at the center.
Also, in the active matrix LC display device, a drain voltage applied when the TFT element is turned on shifts by V.sub.P in the minus-potential direction of gate voltage when the TFT element is turned off, thereby causing a certain amount of potential fall. Namely, being up and down asymmetrical to the reference level and opposing electrode's level, resultantly V.sub.P (hereinafter referred to as `fieldthrough`) is applied to the LC drive voltage as a DC component. When the DC component is applied to the LC drive voltage, the accumulation of charge occurs, therefore causing an image-sticking defect etc.
Such a phenomenon that the DC component is applied to the LC drive voltage may occur in TN type where electric field is applied perpendicularly to the substrate. Its solution is disclosed in Japanese patent application laid-open No. 61-116392 (1986).
In this application, it is proposed that DC voltage applied to LC is corrected by adding a predetermined potential difference (V.sub.T) to a reference level of AC drive signal. Namely, V.sub.P -V.sub.T is given to be up and down symmetrical to the reference level.
However, LC capacitance C.sub.LC, which varies due to the orientation state of LC (degree of inclination of LC molecule to pixel electrode), generally differs in each pixel. The relationship between this capacitance C.sub.LC and a potential fall different .DELTA.V.sub.P of LC charging voltage when gate voltage is turned off is given by expression 1, which is reported by T. Yanagisawa et al., "Japan Display '86", p. 192: ##EQU1##
In this regard, Japanese patent application laid-open No. 5-72997 (1993) describes that the image-sticking defect becomes most unlikely to happen by setting C.sub.LC of B, a minimum capacitance value when the amplitude of LC drive voltage is small, i.e., when LC is oriented parallel to pixel electrode. This is because the accumulation of charge added to drive voltage by DC component becomes large as the drain voltage increases.
Meanwhile, the fieldthrough is caused by parasitic capacitance C.sub.GS between gate and source of TFT element, and by that the respective charges accumulated in LC capacitance, C.sub.LC and accumulation capacitance C.sub.GS when the gate pulse becomes ON are redistributed to the respective capacitance when the gate pulse becomes OFF. In TN type, on the side of opposing electrode a transparent electrode (opposing electrode) is formed on the color layer, therefore electric field generated by the pixel electrode and opposing electrode does not penetrate through the inside of the color layer and the color layer itself is polarized. Therefore, as shown in expression 1, the item of color layer is not included in the fieldthrough .DELTA.V.sub.P.
In contrast with this, in horizontal electric field drive type, the transparent electrode on the color filter forming substrate in TN type does not exist, therefore the electric flux line generated by the pixel electrode and common electrode penetrates through the inside of the color layer. Namely, the fieldthrough .DELTA.V.sub.P is a function of color-layer capacitance C.sub.COLOR, which is given by: ##EQU2##
Also, the color layers (R, G, B) of color filter are formed considering the color purity and chromaticity level of light transmitted through panel. The color layers (R, G, B) are produced by coloring an organic polymer material, such as polyvinyl alcohol or acryl resin, by using a dyestuff or pigment. Therefore, when producing it by, e.g., pigment scattering, the dielectric constants of the color layers (R, G, B) vary, depending on the kind of pigment or the scattering density. Though the thickness H of color layer is set to be greater than 1 .mu.m so as to enhance the color purity, the respective thicknesses of the color layers (R, G, B) are also different each other because the respective transmissivities of the color layers (R, G, B) are different each other. Accordingly, the color-layer static capacitance represented by the product of color-layer dielectric constant and color-layer thickness H is not constant in the color layers (R, G, B) each.
When a same voltage is applied to the color layers (R, G, B) with such characteristics, the fieldthrough .DELTA.V.sub.P results in differing in the color layers (R, G, B) each because the LC static capacitance and the static capacitance of the color layers (R, G, B) are different each other. Namely, different DC components are applied to LC as to the color layers (R, G, B) each, the accumulation of charge occurs within the panel, thereby causing the image-sticking defect that a residual image occurs when displaying another image after displaying the same image for a long time.
On the other hand, Japanese patent application laid-open No. 2-211402 (1990) discloses a technique the thickness and dielectric constant of color layers (R, G, B). This is devised to get correspondence in optical response of the color layers (R, G, B) each, and relates to TN type where the color layer is formed on the transparent electrode. Therefore, its electrode structure and application of electric field are clearly different from those of this invention.
As described above, in the conventional techniques, there is the problem that different DC components are added to LC drive voltage as to the color layers (R, G, B) each, thereby causing the image-sticking defect. This is caused directly by that, in the horizontal electric field drive type active matrix LC display device, electric flux line to drive LC penetrates through the inside of the color layers (R, G, B) because the transparent electrode does not exist on the color filter forming substrate, and that the color layer itself is therefore polarized due to the difference in sum of color-layer static capacitance and LC static capacitance.